High electron mobility transistors (HEMT's) grown on high resistivity silicon (Si) can have degraded performance due to contamination during fabrication. When a III-nitride layer is grown on a high resistivity Si substrate for use in a HEMT structure, acceptor contaminant species can deposit on and/or diffuse to the top surface of the Si substrate, causing a p-type region to form there upon ionization of the acceptors. The p-type region can have a high free hole concentration, resulting in a parasitic conductive channel. The parasitic channel results in parasitic capacitance that reduces the transistor performance at high frequencies.
In addition, the surface of the Si substrate can be contaminated with oxygen, carbon, and other elements. The contaminants and impurities may include species deposited on the surface of the substrate after desorbing from the chamber walls, adventitious carbon-containing species arising from species present in the environment, and/or a native oxide resulting from oxidation of the substrate by oxygen present in the ambient environment. These contaminants and impurities can lead to the formation of a highly defective interface between the Si and the III-nitride layer. This surface contamination and related defects can reduce the quality of the subsequent III-nitride epitaxial layers and structures. This reduced quality causes reduced electron mobility, deteriorating transistor performance, especially at high-voltage and high-current conditions.